Metabolites of Aspergillus ustus. Part 2. Stereoelectronic control in the acid-catalysed hydrolysis of the ortho ester moiety in austalides A–F
Attempted acetylation of the C-13 hydroxy function in austalides B (2) and D (4) with either acetic anhydride and pyridine or 4-dimethylaminopyridine fails. Acetylation occurs under more forcing conditions using acetic anhydride and catalytic amounts of perchloric acid, but the major products are in each case δ-lactone derivatives. It is shown that the formation of a δ- instead of an Îµ-lactone from the ortho ester moiety is favoured because initial cleavage of the C(17)–O(16) bond is stereoelectronically assisted by the two other oxygen atoms, O-30 and O-35, since each has a lone-pair orbital antiperiplanar to this bond.